Research and development have been widely conducted on light-emitting devices that use LED chips in combination with fluorescent materials (such as fluorescent pigments and dyes). In such devices, the fluorescent materials serve as wavelength conversion materials capable of being excited by light emitted from the LED chips and emitting light of a color different from that of the LED chips so that the devices can emit light of a color different from that emitted from the LED chips. Concerning this type of light-emitting devices, there have been commercially manufactured, for example, white light-emitting devices (generally called white LEDs) that use fluorescent materials in combination with blue or violet light-emitting LED chips to produce white light (white light emission spectrum).
As high output power white LEDs have been developed in recent years, research and development on application of white LEDs to lights have been aggressively conducted. For applications such as general lights that require a relatively large light output power, a single white LED is not enough to produce a desired light output power. Therefore, in general, a plurality of white LEDs are mounted on a circuit board to form an LED unit, and the entire LED unit is used to ensure the desired light output power (see, for example, Japanese Unexamined Patent Publication No. 2003-59332 (Patent Literature 1)).
It has also been proposed that an LED unit including a plurality of LED chips and a circuit board on which the respective LED chips are mounted has a structure for efficiently dissipating heat generated at a light-emitting part of each LED chip to outside such that an increase in junction temperature of each LED chip can be suppressed and that a light output power can be increased with an increase in input power (see, for example, Paragraph [0030] and FIG. 6 of Japanese Unexamined Patent Publication No. 2003-168829 (Patent Literature 2), and FIG. 6 of Japanese Unexamined Patent Publication No. 2001-203396 (Patent Literature 3)).
As shown in FIG. 18, the LED unit disclosed in Patent Literature 2 has a metal circuit board 300 that includes a metal plate 301 on which a patterned circuit 303 formed with patterned conductors is formed with an insulating resin layer 302 interposed therebetween, in which heat generated at each LED chip 10′ can be transferred to the metal plate 301 through a heat transfer member 310. Each LED chip 10′ is a blue GaN LED chip including a crystal growth substrate made of an insulating sapphire substrate and a light-emitting part made of GaN compound semiconductor materials and formed on one side of the crystal growth substrate. Each LED chip 10′ is flip-chip mounted on the circuit board 300, and the other side of the crystal growth substrate forms a light output surface.
As shown in FIG. 19, the LED unit disclosed in Patent Literature 3 has a metal circuit board 300 on which each LED chip 10″ is mounted. Each LED chip 10″ has an anode electrode formed on one side and a cathode electrode formed on the other side. Out of the anode and cathode electrodes, the electrode closer to the circuit board 300 is electrically connected to a first conductor plate 312, and the electrode distant from the circuit board 300 is electrically connected to a second conductor plate 313 through a thin metal bonding wire 314. The first and second conductor plates 312 and 313 are respectively bonded to patterned circuits 303 of the circuit board 300.
In a case where the LED unit configured as shown in FIG. 18 or 19 is used for lighting fixtures, the fixture body may be made of a metal, and the metal plate 301 of the circuit board 300 in the LED unit may be thermally coupled to the fixture body so that heat from the LED unit can be efficiently dissipated. In order to ensure lighting surge resistance, at present, a heat radiating rubber sheet such as Sarcon (Registered Trademark) is interposed as a sheet-shaped insulating material (insulating layer) between the fixture body and the metal plate 301 of the circuit board 300. In such a conventional technique, a portion from the light-emitting part of each LED chip 10′ or 10″ to the fixture body has a high level of thermal resistance, and therefore the power input to each LED chip 10′ or 10″ has to be limited such that the junction temperature of each LED chip 10′ or 10″ does not exceed the maximum junction temperature, which makes it difficult to increase the light output power.
When the heat radiating sheet is interposed between the metal plate 301 of the circuit board 300 and the fixture body, the heat radiating sheet may be insufficiently bonded to the metal plate 301 so that a void may be formed between the metal plate 301 and the heat radiating sheet to increase the thermal resistance or to cause variations in thermal resistance of the portion from the light-emitting part of each LED chip 10′ or 10″ to the fixture body.
In the LED unit disclosed in Patent Literature 2, the heat generated at the light-emitting part of the LED chip 10′ is transferred to the metal plate 301 through the heat transfer member 310 that is smaller in size than the LED chip 10′. Therefore, the portion from the LED chip 10′ to the metal plate 301 has a relatively high level of thermal resistance. When the sapphire substrate serving as a crystal growth substrate is mounted so as to be thermally coupled to the metal plate 301, there is also a problem in which the sapphire substrate provides high thermal resistance.
The present invention has been made in light of the circumstances described above, and it is an object of the present invention to provide an LED lighting fixture in which an increase in temperature of an LED chip can be suppressed so that a light output power can be increased.